Huntington's disease (“HD”) is a degenerative brain disorder. It slowly destroys an affected individual's ability to walk, think, talk and reason. Symptoms include changes in cognitive ability, such as impaired short-term memory and a decreased ability to concentrate; changes in mood, such as the development of mood swings, depression and irritability; and changes in coordination and physical movement such as clumsiness, involuntary movements and twitching. These symptoms gradually worsen until HD patients die, approximately 15-20 years after the onset of the disease.
While the biochemical cause of HD is not yet fully understood, it is now known that HD is inherited. Organisms typically inherit two copies of each gene, one from each parent. An individual copy of a gene inherited from one parent is called an allele. With regard to the inheritance of HD, every individual who inherits a HD-causing allele from either parent will develop the disease.
One breakthrough in research regarding HD has been the identification of the mutated gene that causes HD. Based on this breakthrough, researchers and physicians now can predict which individuals will develop HD. Specifically, researchers and physicians can predict which individuals will develop HD by counting the number of “CAG repeats” that exist within an individual's HD genes. If a person has 35 or less CAG repeats in each of their HD gene alleles, that person will not develop HD. If a person has more than 35 CAG repeats in either of their HD gene alleles, that person will develop the disease. The more CAG repeats a person has over 35 in either gene allele, the earlier the person will develop the symptoms of HD.
The HD gene encodes for a protein called “huntingtin” (“hit”). The exact function of htt is not known. However, it is known that the increased number of CAG repeats in HD-causing gene alleles leads to htt proteins with expanded polyglutamine sequences. Expanded polyglutamine sequences in proteins confer novel toxic properties resulting from a tendency of the protein to misfold and form aggregates within brain cells. Thus, suppressing the production of htt in brain cells may provide an avenue to prevent the accumulation of toxic htt in brain cells. This prevention can provide an avenue to further study the physiological mechanisms underlying HD and may also prevent or alleviate the symptoms or occurrence of HD.
While suppressing the production of htt could provide a method to prevent or treat the symptoms of HD, because its functions are not fully understood, fully blocking the expression of this protein may not be desirable. Indeed, animal studies have shown that htt is critical at least during development because animals lacking the protein altogether do not survive. Thus, it would be desirable to preferentially suppress the expression of toxic or mutated htt encoded for by a mutated HD allele while allowing expression of the normal HD allele that encodes for a normal htt protein.
Recent developments in genetic technologies have made the selective suppression of certain alleles and proteins, such as the mutated HD gene allele and the mutated htt protein, possible. Some background in the art is required to understand the potential impact of these technologies. Generally, for a protein to exert an effect, the cell that will use the protein must create it. To create a protein the cell first makes a copy of the protein's gene sequence in the nucleus of the cell. This copy of the gene sequence that encodes for the protein (called messenger RNA (“mRNA”)) leaves the nucleus and is trafficked to a region of the cell containing ribosomes. Ribosomes read the sequence of the mRNA and create the protein for which it encodes. This process of new protein synthesis is known as translation. A variety of factors affect the rate and efficiency of protein translation. Among the most significant of these factors is the intrinsic stability of the mRNA itself. If the mRNA is degraded quickly within the cell (such as before it reaches a ribosome), it is unable to serve as a template for new protein translation, thus reducing the cell's ability to create the protein for which it encoded.
Based on the foregoing, the technology of RNA interference (“RNAi”) has emerged. RNA interference is, in fact, a naturally occurring mechanism for suppressing gene expression and subsequent protein translation. RNA interference suppresses protein translation by either degrading the mRNA before it can be translated or by binding the mRNA and directly preventing its translation. Generally, RNAi has been used to suppress the expression of both gene alleles that lead to the production of a given protein. However, in some cases, such as those involved in the present invention, it is desirable to suppress the expression of only one allele.
Recently, several single nucleotide polymorphisms (“SNPs”) have been identified that can be used to distinguish between the two HD gene alleles in human cells. SNPs are single nucleotide changes in the nucleotide sequence of a given gene. Thus, using RNAi, it is now possible to derive therapeutic agents to specifically suppress the expression of a mutated HD gene allele carrying a SNP. This personal approach can lead to individualized or “directed” therapeutics for HD patients, and provides an avenue to suppress the expression and actions of mutated toxic htt in an attempt to prevent or alleviate the symptoms of HD.